Sealing materials are used, for example, for oxygen sensors (O2 sensors). The oxygen sensors are used, for example, for detecting an oxygen concentration in exhaust gas from internal-combustion engines of vehicles. Oxygen sensors for vehicles are generally protected by metals or the like. However, the sensors are attached to hot exhaust pipes or the like, and the inner portion of the sensors is exposed to hot exhaust gas flows containing a large amount of oxidizing substances. Therefore, the sensors need to have heat resistance, chemical resistance, and the like.
The oxygen sensors for vehicles are commonly mounted on a lower side of vehicle body floors. Examples of the sensors include sensors mounted at a lower stream of catalysts that purify exhaust gas, and sensors required to detect degradation of catalysts. In this case, the oxygen sensors for vehicles are subjected to external impacts. For example, the sensors are subjected to vibration impacts from engines and road surfaces, hit with stones, and splashed with water. Therefore, the oxygen sensors need to have mechanical shock resistance, thermal shock resistance, and waterproof properties.
The oxygen sensors for vehicles generally have a tubular shape, and have several leads for introducing air, which is reference of an oxygen concentration, into an oxygen concentration detection part, and taking away electric generating power of an oxygen-concentration detecting element located in the deep in the tube. In order to fix the leads without contacting one another at a portion of the oxygen sensors from which the leads are taken out, the leads are arranged so as to penetrate a sealing material, which is called a bush.
The sealing materials generally have a pillar shape basically, and have several previously-formed through holes that extend in a height direction of the pillar. When using such a sealing material, the leads are allowed to pass through the through holes, pressure in a diameter direction is applied to the sealing materials, and the sealing materials are crimped.
The sealing materials are compressed to some extent by crimping, thereby securely fixing the leads. The sealing materials desirably have elasticity so that sealability such as waterproof properties and airtightness is exhibited. In view of the setting position, similarly to the main body of oxygen sensors for vehicles, the sealing materials further desirably have the characteristics such as heat resistance and impact resistance. Accordingly, sealing materials produced by cross-linking fluororubber compositions comprising fluororubbers with the above characteristics have been commonly used.
The sealing materials easily generate cracks if stress deformation caused by crimping remains permanent therein, and therefore desirably have as low compression set as possible for showing excellent sealing characteristics. In conventional fluororubber compositions, cross-linking density is increased by an increase in an amount of cross-linking agents, which reduces compression set of the sealing materials. In this case, however, the sealing materials are highly compressed, which deteriorates crack resistance thereof. Conversely, cross-linking density is reduced by a decrease in an amount of cross-linking agents, which improves crack resistance of the sealing materials. In this case, however, the compression set of the sealing materials is increased. Therefore, adjustment of the amount of cross-linking agents can not satisfy both compression set resistance and crack resistance.
In order to solve such problems, Patent Document 1 discloses a rubber composition in which a compounding ratio of various additives to be added to fluororubber is defined. However, such a rubber composition cannot sufficiently satisfy compression set resistance and crack resistance at the same time.
Further, Patent Document 2 suggests a sealing material comprising a composition including a fluoroelastomer. The fluoroelastomer is a copolymer of monomer components including vinylidenefluoride, tetrafluoroethylene, and hexafluoropropylene and has an average molecular weight of 400,000 to 700,000. However, there has been a problem that compression set resistance of the sealing material is insufficient, and in view of the processability of a fluororubber or a fluororubber composition, flowability is too poor to measure Mooney viscosity, and therefore the fluororubber and the fluororubber composition are not substantially processable. Further, Patent Document 3 suggests a rubber composition comprising a fluororubber and a cross-linking agent, and having a Mooney viscosity (ML1+20, 140° C.) of 70 to 150. Patent Document 4 suggests a cross-linked and formed product containing a fluororubber and a thermal black. However, the compression set resistance of the sealing materials is still insufficient, and there is room for improvement in crack resistance.
Further, sensor control with high accuracy has been needed as performance of engines and devices is improved, and environmental conservation awareness is improved. The oxygen sensors for vehicles these days are mounted on the upstream region of an exhaust gas flow whose temperature is higher in the upstream.
Further, downsizing of engines has been needed for creating a large residence space of vehicles along with the density growth of engine rooms. The development of downsized engines has been highly needed because of its contribution to lightening of the weight of vehicles and lowering of fuel consumption. However, the downsized engines cause elevation of the temperature inside the engine room. Some oxygen sensors may be placed at such a portion with high temperatures.
Thus, the crack resistance and the compression set resistance under high compression tend to be lowered at elevated temperatures. Therefore, fluororubbers used for sealing materials have been needed not to impair such characteristics under high temperature use conditions compared with conventional use conditions.
Patent Document 1: JP 9-188793 A
Patent Document 2: WO 2003/074625
Patent Document 3: WO 2006/040944
Patent Document 4: JP 2001-192482